Hitherto, there are many cases where small-sized blades, such as turbine blades, paddle blades and propeller blades are disposed at multiple stages when in use as a stirring impeller of a stirring apparatus. According to a stirring apparatus using these blades, the rate of the fluid flow pumped in the radial direction from the blades is increased by increasing the rotational speed of the impeller, so that the fluid strikes against a tank wall, and then turns upwards and downwards, thereby creating a circulation flow returning again to the impeller, and thus mixing the inside of the tank.
However, the small-sized blades hardly create a satisfactory vertical circulation flow throughout the inside of the tank, as illustrated in FIG. 14, and thereby creating plural isolated zones in the vertical direction, so that the stirring characteristics cannot be improved as expected. In addition, these impellers tend to have complicated shapes, which causes increase in manufacturing costs and makes them difficult to wash.
Contrarily to this, a flat plate type impeller disclosed in, for example, Patent Document 1 has a simplified impeller shape, and thus has merits of lowering the manufacturing cost and allowing for ease of washing, as well as a merit of producing good stirring characteristics because of a high rate of the fluid flow pumped in the radial direction from the impeller throughout the inside of the tank, so that it is suitable for stirring treatment of particularly highly viscous fluid, and fluid containing solid matters that are easy to precipitate onto a tank bottom.    Patent Document 1: Japanese Unexamined Patent Application Publication No. Hei-10-174857